1. Field of the Invention
The present invention relates to white inkjet ink printing methods.
2. Description of the Related Art
Printing systems, such as offset and flexography, are being increasingly replaced for printing applications, such as billboards, packaging and decoration, by industrial inkjet printing systems due to their flexibility in use, e.g. variable data printing, and due to their enhanced reliability, allowing their incorporation into production lines. UV curable inkjet inks are particularly preferred because high quality images can be printed on non-absorbing ink-receivers.
White UV curable inkjet inks are advantageously used to mask defects on the ink-receivers and enhance the contrast and vividness of colour inkjet inks in a so-called “surface printing mode” or in a “backing printing mode”. In surface printing, a colour image is printed on a white background formed on an ink-receiver using white inkjet ink, hereafter the final image is viewed from the printed face. In so-called backing printing, a colour image is printed on a transparent ink-receiver using colour inkjet inks and then a white inkjet ink is applied onto the colour image, hereafter the final image is observed through the transparent substrate.
For a white inkjet ink, preferably pigments with a high refractive index, such as titanium dioxide, are used in white ink in order to obtain a sufficient opacity. Sedimentation of these dense particles in a low viscosity fluid, such as an inkjet ink, is a real challenge for ink formulators. Problems of clogging of inkjet print head nozzles and poor storage stability of the ink are direct consequences of sedimentation and aggregation of white pigments due to the difference in specific gravity between pigment particles and the liquid medium of the ink. Hence, improvement of inkjet printing methods employing white inkjet ink for reliability and incorporation into production lines is still necessary.
One approach is to reduce sedimentation of the white inkjet ink. For example, U.S. Pat. No. 4,880,465 (VIDEOJET) discloses a non-pigmented white inkjet ink comprising hollow microspheres in the ink vehicle. Sedimentation is drastically reduced, however the opacity of such a printed white layer remains rather limited. EP 2599841 A (AGFA) discloses a white inkjet ink prepared by combining a white pigment dispersion having an average particle size above 200 nm with a second pigment dispersion having an average particle size between 40 and 90 nm. White ink sedimentation was not prevented, but it took only minor force to redisperse to a very large extent the sediment of the white pigment at the bottom of a stored ink container. EP 1770132 A (FUJIFILM) discloses an inkjet-recording ink composition comprising one or more white pigment(s), one or more polymer dispersant(s), one or more photocurable compound(s), and one or more photoinitiator(s) as its principal components, wherein the polymer dispersant(s) has a sulfonic acid group. The white inkjet ink has a composition that prevents nozzle clogging by improving dispersion stability of the ink and suppresses sedimentation of the pigment.
Another approach is to use specific inkjet printing methods or inkjet printer configurations. For example, EP 2053099 A (AGFA) discloses a inkjet printing method comprising the step of jetting a higher viscous white inkjet ink at a higher temperature than a colour inkjet ink. It is known that less sedimentation of white pigment is observed when the white ink possesses a higher viscosity. The effect of a higher viscosity for less sedimentation is also exploited in inkjet printing methods WO 2008/074548 (AGFA), where a higher viscous white ink is mixed inside the inkjet printer with one or more colourless liquids to form a white ink-mixture having a viscosity suitable for inkjet printing.
Problems of white inkjet ink sedimentation lead to frequent maintenance of white inkjet print heads using so-called “flushing” or “washing” liquids for unclogging of inkjet nozzles and cleaning the nozzle plate of the print head. For example, EP 2157163 A (TOYO INK) discloses a maintenance liquid for inkjet printers comprising at least one of glycol ethers and glycol esters and 45 to 10 mg/L of dissolved oxygen. EP 1621348 A (TOSHIBA TEC) discloses a washing solution for washing a cationically UV curable inkjet ink inkjet printer head, which contains not less than 50 parts by weight of a polymerizable compound selected from the at least two kinds of polymerizable compounds included in the ink and having the lowest viscosity among the at least two kinds of polymerizable compounds, or not less than 50 parts by weight of a polymerizable compound having a viscosity of 30 mPa·s or less at ordinary temperature.
However, there is still room for improvement of white inkjet inks and white inkjet ink printing methods wherein the problems of nozzle clogging and sedimentation are reduced so that reliable inkjet printing of white ink in industrial environment is possible.